The present invention relates to thermoforming presses and more particularly to presses used to form three-dimensional shaped articles from polymeric sheet stock.
Various types of presses are presently available for forming shaped articles, such as containers, from polymeric sheet stock. One such type of press is utilized in a process called thermoforming. It uses a mold, or multiplicity of molds, each having a peripheral cutting knife. The upper platen of the press supports the molds and is movable while the lower platen is a flat surfaced heating platen attached to the lower stationary bed of the press. This lower heating platen has small air holes which communicate a source of air pressure or a vacuum. Thus the polymeric sheet is placed on the platen, the molds are lowered until the knives are slightly embedded in the polymeric sheet, and the vacuum is connected to the heating platen. While the vaccum holds the sheet against the heating platen, the sheet is heated to a plasticized state. The vacuum is then removed and air pressure is applied through the air holes causing the polymeric sheet to stretch and be forced into the shape of the mold on the upper platen.
In some instances, the cavity of the mold is subjected to vacuum in which case the vacuum causes the polymeric sheet to be formed in the mold. This process would be called vacuum forming whereas applying pressure through the air holes is called pressure forming. When the plasticized sheet contacts the mold surface it is relatively cooled and sets in the form of the mold. At this time the press closes slightly further and the knives cut through the polymeric sheet after which the press is opened and the three-dimensionally shaped article is removed.
An example of one such press may be found in U.S. Pat. No. 3,340,574 entitled UNIVERSAL FORMING PRESS and issued on Sept. 12, 1967, to O'Brien et al. The forming press disclosed in this aforementioned patent includes a frame structure supporting an upper and a lower platen. The upper platen is suspended on a toggle linkage. A piston cylinder attached directly to the toggle linkage shifts the upper platen towards and away from the lower platen.
Presses of the aforementioned type have significant height and space requirements. These requirements result in installation problems and limit the facilities which can be used. Also, significant power requirements are experienced to raise and lower the heavy, movable platen and actuating linkages. Some presses employ only hydraulic pressure to hold the dies in position during the forming operation. Significant pressures are developed between the die and the lower platen, and relatively large piston cylinder assemblies are required to hold the die shut against the lower platen.
A need exists for a forming press and actuating mechanism therefor whereby the height and space problems heretofore experienced are eliminated, which is capable of raising and lowering a movable die and holding the die in forming position with a significant reduction in power requirements and which is adaptable to pneumatic or mechanical operation.